The present invention relates to medical imaging arts. In particular, it relates to a rotating gantry such as those found in 3rd and 4th generation CT scanners, and will be described with particular reference thereto. However, the invention will also find application in conjunction with nuclear cameras and other imaging systems with rotating bearings, and is not limited to the aforementioned application.
Typically, 3rd and 4th generation CT systems have rotating gantries and stationary gantries. The two gantries are interfaced by a bearing system that allows rotation of the first gantry relative to the second gantry.
A large ball bearing assembly, often a meter or more in diameter, has been used to provide the interface between the gantries. Large ball bearing assemblies are expensive and tend to be noisy.
In other systems, roller bearings have been used. Cylindrical rollers support the rotating gantry in both axial and radial directions. Typically, the rotating gantry has three bearing races, or tracks along which the bearings roll. A circumferential race allows the bearings to give the rotating gantry radial support (a normal force counteracting the force of gravity) while the second and third races allow bearings to give the rotating gantry lateral, that is, axial support. To prevent the rotating gantry from wobbling, the roller bearings press against the second and third races with significant opposing pressure.
While the gantry rotates with a constant angular velocity, portions of the gantry move with different radially dependent linear velocities. More specifically, portions of the second and third races more distant from the rotational axis of the gantry have a higher linear velocity than portions closer to the rotational axis. Stated differently, the linear velocity of any moving element is a function of radial position, as well as angular velocity of the gantry.
This is significant to, among other things, the second and third axial support bearing races. The outer edges of these two races move faster than the inner edges of the same races. Each cylindrical roller bearing that contacts the second and third races only rotate at a single speed. Thus, slippage occurs between the bearing races and the roller bearings, causing high friction and wearing both the bearing races and the bearings prematurely. Additionally, functional speeds of the gantry are limited, in order to balance the speed of the gantry and the wear that higher speeds incur on the races and the bearings.
The present invention contemplates an improved apparatus and method, which overcomes the aforementioned limitations and others.